This invention relates generally to asymmetric digital subscriber loop (ADSL) transceivers and more particularly to discrete multi-tone (DMT) asymmetric digital subscriber loop (ADSL) transceivers.
As is known in the art, ADSL modems achieve full-duplex operation over a single pair of subscriber loop (i.e., twisted pair telephone line) through the use of either frequency-division-multiplexing (FDM) or echo cancellation (EC). ADSL modems use DMT, a multi-carrier modulation technique, to achieve high bandwidth efficiency over a 1.1 Mhz bandwidth. An ADSL transceiver system generally includes a modem at a central station, or office, adapted to transmit information in a downstream signal to a modem at a remote terminal and to receive information in an upstream signal transmitted by the modem at the remote terminal. The upstream and downstream signals pass through a common transmission medium, typically the twisted-pair telephone line. The upstream signal and the downstream signal have different (i.e., asymmetric) bandwidths, the ratio of the downstream signal bandwidth to the upstream signal bandwidth being K, where K is greater than one. More particularly, with the current ANSI ADSL standard, the downstream signal bandwidth is eight times wider than the upstream signal bandwidth; i.e., K=8. This frequency bandwidth asymmetry is intended to accommodate a large downstream signal data rate to support data-hungry applications such as video-on-demand and Internet access, and a low upstream signal data rate for interactive control and basic--rate IDSN. In terms of the multi-carrier modulation, the downstream band consists of carriers 1-255 whereas the upstream consists of only carriers 1-31. In actual practice, the first 6 to 8 carriers are used as a guard-band for plain ordinary telephone service (POTS). An echo-cancellation based system makes use of the carrier allocation just mentioned above, however, FDM systems avoid the overlap of upstream and downstream by using a carrier assignment such as 32-255 for downstream and 8-28 for upstream, for example.
One DMT FDM ADSL system 10, shown in FIG. 1, is adapted to exchange information between a modem 12 at a first station, here a central office (CO), and a modem 14 at a second station, here a remote terminal (RT), through a common communication medium 16, here a twisted-pair telephone line. The system 10 includes: a transmitter section 18, at the central office modem 12, for distributing a first stream of data on line 13 among a plurality of, P, carriers. More particularly, the transmitter section 18 of the central office modem 12 includes a modulator 20 for receiving frames of the data on line 13 and for distributing such data over carriers 32 through 255. The modulator 20 includes a Quadrature Amplitude Modulation (Q.A.M.) encoder 24 and an Inverse Fast Fourier Transformer (I.F.F.T.) 26 arranged in a conventional manner as shown. The I.F.F.T. 26 is an N point, here 512 point I.F.F.T. Thus, the incoming data on line 13 is selectively encoded by the Q.A.M. encoder 24 at a frame rate, f.sub.r, here about 4 kHz (more precisely, 4.0588 Khz) and the I.F.F.T. 26 produces, for each frame, a sequence of digital samples on line 22 at a rate f.sub.s =N f.sub.r where here N is 512. More particularly, the sequence of digital samples on line 13 is encoded by Q.A.M. encoder 24 onto the 255 conjugate data pairs on input lines 28.sub.1 -28.sub.512 of the I.F.F.T. 26 as a sequence of frames, here at a frame rate of about 4 Khz. It should be noted that the I.F.F.T. 26 has 512 frequency bins a pair of which correspond to each one of the 255 carriers for a total of 510 bins with the remaining two bins, i.e., bins 1 and 257, corresponding to DC and f.sub.s /2, respectively. Each carrier is associated with a pair of input lines on bus 28.sub.1 -28.sub.512. Here, carrier 1, at 4.3125 kHz, is associated with the conjugate data on lines 28.sub.2 and 28.sub.512, carrier 2 is associated with the conjugate data on lines 28.sub.3 and 28.sub.511, carrier 3 is associated with the conjugate data on lines 28.sub.4 and 28.sub.510, . . . , carrier 255, corresponding to 1.1 MHz is associated with the conjugate data on lines 28.sub.256, 28.sub.258. Thus, for each frame of data fed to lines 28.sub.1 -28.sub.512 a sequence of digital samples is produced by the I.F.F.T. 26 on line 22 at a sampling rate of about f.sub.s =2.208 MHz. As noted above, here the FDM system 10 will transmit only carriers 32-255 in the downstream signal and carriers 8-28 in the up-stream signal.
The transmitter section 18 of the central office modem 12 also includes: a digital to analog converter (DAC) 30 for converting the sequence of samples of digital samples into a corresponding analog signal on line 32 at a rate f.sub.s ; and a band pass filter 34, fed by the analog signal and having a pass band extending over carriers 32-255 for producing, after passing through a conventional isolation hybrid 36 the downstream signal on the common communication medium 16 with a bandwidth extending over the P carriers 32 through 255.
The remote terminal modem 14 includes a receiver section 40 having: a band pass filter 42, coupled to the common communication medium 16 via a hybrid 37, for passing signals in the downstream signal fed thereto by the central office modem 12 transmitter section 18. As noted above, the data in the downstream signal extends over the P carriers 32 through 255). An analog to digital converter (ADC) 44 is provided for converting the signals passed by the band pass filter 42 into a sequence of digital data on bus 46. The data on line 46 is produced at the sampling rate, f.sub.s. The data on line 46 is fed to a demodulator 47. The demodulator 47 includes a Time Domain Equalizer (T.D.Q.) 50 followed by a Fast Fourier Transformer (F.F.T.) 48 followed by a Frequency Domain Equalizer (T.D.Q.) 49 and Q.A.M. demodulator 45. The F.F.T 48 is fed by the sequence of data samples produced by the T.D.Q. 50 and separates such digital data fed thereto into P carriers 32 through 255. More particularly, the F.F.T. 48 provides a 512 point transform on the analog signal produced on line 46 by the ADC 44 to separate the analog signal on line 46 into 255 frequency bins. The conjugate data on pairs of lines 51.sub.2, 51.sub.512 correspond to carrier 1, the conjugate data on lines 51.sub.3, 51.sub.511 correspond to carrier 2, . . . , the conjugate data on pairs of lines 51.sub.33 and 51.sub.481 correspond to carrier 32, the conjugate data on pairs on lines 51.sub.34 and 51.sub.480 correspond to carrier 33, . . . and, the conjugate data on pairs of lines 51.sub.256 and 51.sub.258 correspond to carrier 255. As noted above, the downstream signal will only be in carriers 32-255. The data on lines 51.sub.33 -51.sub.256 (corresponding to carriers 32-255) is processed by the F.D.Q. 49 and Q.A.M. demodulator 45 into a signal on line 54 which, ideally, corresponding to the stream of data on line 13.
The remote terminal modem 14 includes a transmitter section 60, for distributing a second stream of data fed to the remote terminal modem 14 on line 63 among the P' carriers 8 through 28. More particularly, the transmitter section 60 includes a modulator 62 for receiving data on line 63 and for distributing such data over carriers 8 through 28. The modulator 62 includes a Quadrature Amplitude Modulation (Q.A.M.) encoder 64 and an Inverse Fast Fourier Transformer (I.F.F.T.) 66 arranged in a conventional manner as shown. The I.F.F.T. 66 is a 64 point I.F.F.T. Thus, the I.F.F.T. 66 produces for each frame on line 63 arranged a sequence of digital samples at a rate f.sub.s =Nf.sub.r /8. More particularly, a sequence of digital samples on line 63 is encoded by Q.A.M. encoder 66 as a sequence of frames, here at a frame rate of about 4 kHz. It should be noted that the I.F.F.T. 66 has 31 frequency bins, or carriers. Each carrier is associated with a pair of input lines 68.sub.1 -68.sub.64. Here, carrier 1, corresponding to 4.3125 kHz, is associated with the conjugate data on lines 68.sub.2 and 68.sub.64, carrier 2 is associated with the conjugate data on lines 68.sub.3 and 68.sub.63, carrier 3 is associated with the conjugate data on lines 68.sub.4 and 68.sub.62, . . . , carrier 31 is associated with the conjugate data on lines 68.sub.32, 28.sub.34. Here, however, the upstream signal includes only carriers 5 through 28. For each frame of data fed to lines 68.sub.1 -68.sub.64, a sequence of digital samples is produced by the I.F.F.T. 66 on line 70 at a rate of f.sub.s /8. It is noted that the sequence of digital samples fed to each one of the lines 68.sub.1 -68.sub.64 at the approximately 4 kHz frame rate f.sub.r appear in one of the 31 carriers of the upstream signal, it being understood that only carriers 8-28 are used in the F.D.M. system.
The transmitter section 60 of the remote terminal modem 14 also includes: a digital to analog converter (DAC) 72 for converting the sequence of samples of digital samples produced by I.F.F.T. 66 into a corresponding analog signal on bus 94. A lowpass filter 96 is fed by the analog signal and has a bandwidth extending over carriers 1 through 28, for producing, after passing through a conventional isolation hybrid 37, on the common communication medium 16, the upstream signal having a bandwidth extending over carriers 8 through 28.
The central office modem 12 includes a receiver section 80 having: a lowpass filter 82, coupled, via the isolation hybrid 36, to the common communication medium 16, for passing signals in the upstream signal fed thereto by the remote terminal modem 14 extending over the P carriers 8 through 28. An analog to digital converter 84 is provided for converting the signal passed by the lowpass filter 82 into a sequence of digital data on line 86 at the sampling rate, f.sub.s /8. A demodulator 91 is fed the data on line 86. The demodulator 91 includes a Time Domain Equalizer (T.D.Q.) 85, a 64-point F.F.T. 92, a F.D.Q. 93 and a Q.A.M. demodulator 97. The F.F.T. 92 separates the analog signal on line 86 into 31 frequency bins, or carriers on lines 95.sub.1 -95.sub.64, it being understood that only signals in carriers 8-28 are allocated in the FDM system. The F.F.T. 92 operates at a rate f.sub.s /8, to produce frames of data at the approximately 4 kHz frame rate among carriers 1 through 31; it being understood that the data of interest will appear in the P' carriers 8 through 28 for the FDM system example. That is, the F.F.T. 92 separates the signal on line 86 into the carriers 1-31 (it being understood that only carriers 8 through 28 are of interest). The F.D.Q. 93 and Q.A.M. demodulator 97 process the data produced by the F.F.T. 92 into a signal on line 94 which, ideally, corresponds to the data on line 63.
It should be noted that because the transform size (or dimension) used for the upstream signal (i.e., a 64 point transform) is different from the transform size used for the downstream signal (i.e., a 512 point transform), the downstream signal bandwidth of 255 carriers extends to f.sub.s /2 and the upstream signal bandwidth of carriers 8-28 extends to nearly f.sub.s /16. Thus, considering first the upstream signal produced by the remote terminal modem 14, it is first noted that the upstream signal has a bandwidth from carrier 8 through 28. However, the data produced on line 70 by modulator 62 also includes images which repeat at the rate of data are produced on line 70; i.e., at a rate f.sub.s /8. Thus, images of carriers 8-28 repeat at the frequency f.sub.s /8. It is noted that such images extend into the bandwidth of the downstream signal, i.e., into carriers 32 through 255. These un-wanted images are filtered, to some extent, by the upstream transmit low-pass filter 76 and the hold effect of the DAC 72, but they are not removed altogether. The amount of filtering is limited since the use of high-order analog filters for filter 96, in addition to being expensive, also have long impulse responses that introduce excessive intersymbol interference and degrade modem performance. The remaining images pass through the hybrid 37 with some limited attenuation, and appear, as noted above, directly within the band (i.e., carriers 32 through 255) occupied by the downstream signal thereby causing interference. It is noted that the attenuation in the downstream signal increases at higher signal frequencies so that the echo signal from the DAC 72 images can exceed the level of the received downstream signal. As a result, severe degradation of the downstream signal to noise ratio (SNR) may result.
A similar problem occurs at the central office modem 18. At the central office modem, the ADC 84 sampling rate, f.sub.s /8, is one-eighth that of the sampling rate, f.sub.s, of the downstream signal. That is, samples are produced by ADC 84 on bus 86 at a rate, f.sub.s /8, whereas samples are produced by the downstream DAC 30 at the rate f.sub.s. It is noted that these images, centered at multiples of the upstream sampling rate, f.sub.s /8, of the ADC 84 occupy the frequency spectrum of carriers 32 through 255, i.e., the frequencies of the downstream signal. Clearly, with limited hybrid 36 attenuation, the downstream signal will be aliased by the ADC 84 and will fall directly within carriers 8-28 causing interference with the upstream signal.